1. Field of the Invention
The present invention relates to a starter that starts an engine.
2. Description of the Related Art
Conventionally, for the above type of starter, a direct-current (dc) motor is adopted as a starting motor. There is a possibility that brush wear debris may be accumulated on the internal surface of a bracket and a leakage may occur between brush springs, which press respective positive brushes to a commutator or constrain the respective positive brushes to move toward the commutator, and the bracket. Therefore, a gap of a certain length or more has to be preserved between the brush springs and bracket. This imposes a limitation on a brush length and hinders durability. If the starting motor is continuously supplied with electricity for some reason, the commutator may be broken due to centrifugal force and a thermal effect. At this time, if the positive brushes, and the metallic brush springs and brush holders of the respective positive brushes are brought into contact with the internal surface of the bracket and short-circuited, there is a fear that a large current may flow upstream from the short-circuited point (for example, to a connector outside the starting motor) so as to bring about a terrible thermal damage.
As one of countermeasures, it has been proposed as described in, for example, JP-A-10-191589 that an insulating plate is attached to the inner circumferential surface of a frame, which is opposed to a brush unit, in order to ensure an insulation resistance for the frame of a dc motor and brushes.
However, the structure described in JP-A-10-191589 is such that a lead to be coupled to an external power supply is led out from the frame. The led part of the lead cannot be insulated. The aforesaid short-circuit fault cannot be therefore coped with. In addition, since an insulating plate is disposed on the internal surface of the frame so that it will be shaped like a letter C, the gap between the frame and brush springs decreases. The length of the brushes has to be decreased in consideration of assembling efficiency or the like. This degrades durability.